Apparatus for controlling contrast in electrostatic printers



June 10, 1969 I D. A. STARR, JR 3,449,753

APPARATUS FOR CONTROLLING CONTRAST IN ELECTROSTATIC PRINTERS Filed Feb. 1, 1965 Sheet of 2 II II Ill.

lilll INVENTOR AGENT DAYID A. STARR,JR.

June 10, 1969 I D. AISTARR, JR 3,449,753

APPARATUS FOR CONTROLLING CONTRAST IN ELECTROSTATIC PRINTERS Filed Feb. 1, 1965 Sheet 3 of 2 Fig. 5

105 Fig. 4 101- 103 United States Patent ce US. Cl. 346-74 4 Claims ABSTRACT OF THE DISCLOSURE A device for controlling the contrast or density of characters produced by an electrostatic printing assembly formed with a plurality of printing electrodes and ionization initiation electrodes arranged adjacent to a backing electrode to define a gap through which a dielectric recording medium is passed. The printing contrast is a function of the electric field established in the gap and is controlled by varying the bias potential on the backing electrode with respect to the printing and initiation electrodes, without affecting the bias and information signal potentials applied between the printing and initiation electrodes. Additional switching circuitry is provided for reversing the electric field polarity to instantly extinguish electric charge deposition on the recording medium and thereby further improve printing contrast.

This invention relates to electrostatic printing and, more particularly, to apparatus for controlling contrast during electrostatic printing without altering the amplitude of the coincident voltage necessary for initiating a selected pinbar discharge.

The electrostatic recording process comprises the steps of printing, inking, and fixing. In the printing step, indicia to be recorded are formed as shaped latent electrostatic images on a high resistivity surface of a record sheet or web. Subsequently, during the inking step, these previously deposited charged areas are rendered visible by the application of a finely powdered develop-ing agent or ink which is attracted to the previously charged areas and retained thereover by electrostatic attraction. The third step, which is optional, comprises fixing the inking powder adhering to the latent images on the record sheet and may comprise the steps of heating a specially coated record sheet and subjecting the powdered images thereon to a rolling pressure contact.

This invention is a further development of and an improvement upon the electrostatic recording apparatus disclosed in US. Patent No. 3,068,479, granted Dec. 11, 1962, to R. E. Benn et al., and assigned to the assignee of the present invention. A multi-element electrostatic recorder is disclosed in this patent including initiating and printing electrode-s collectively constituting an assembly spaced from a backing or anvil electrode to form a gap into which a dielectric recording surface is introduce-d.

In electrostatic transducers having initiating and printing electrodes, the printing step is commenced by caus ing ionization to occur between a selected initiating electrode and its associated printing eelctrode by applying coincident voltage pulses to the respective electrodes sufiicient to cause ions to be generated therebetween. The printing potential is commonly the combination of a fixed bias, which is applied in parallel across the respective initiating and printing electrodes, and print initiating pulses which are applied to selected initiating and printing electrodes.

The print initiating ionization which results from the application of the print initiating pulses, introduces electrically charged particles or ion-s into the space between 3,449,753 Patented June 10, 1969 the print head electrodes and the backing or anvil electrode which supports a record medium. As explained in the aforesaid patent the charged particles thus generated propagate across the printing gap under the influence of an electric field which exists in the gap due to the fixed bias applied across the printing and backing electrodes respectively. The size, shape, and charged density of an electrostatic image formed on a record medium as the result of an electrical discharge between selected initiating and printing electrodes depends on a combination of parameters including the size and shape of the respective electrodes, the distance of these electrodes from the dielectric recording medium, the polarity of the voltage applied to the respective electrodes and the electric field intensity in the gap between the printing electrode and the backing electrode at the time the cumulative ionization commences.

It is desirable in any printing operation to be able to control the density of the deposited charge independently of those parameters whose values are imposed by consideration of the factors altecting the reliability of initiating ionization.

The electric field intensity in the gap between the printing electrode and the backing electrode could be varied by altering the bias applied between the respective printing electrodes and the backing electrode. Further, either the amplitude of the bias applied to the initiating and printing electrodes or of the print initiating pulses could be varied for altering the potential of the discharge across the printing gap. However, in order to obtain reliable electrostatic recording, it is highly desirable that the amplitude of the coincident pulse potential applied to the respective initiating electrodes not be altered as this could result in the unreliable initiation of printing ionization It is, therefore, an important object of this invention to provide an improved method for controlling the density of charge deposited on a record medium during electrostatic printing.

It is an import-ant object of this invention to provide an improved method and apparatus for electro'statical-ly recording images of variable charge density on a record medium.

It is a further object of the present invention to provide a method for varying the potential difference between predetermined electrodes of an electrostatic printer while simultaneously maintaining a constant bias across electrodes associated therewith.

It is a still further object of the present invention to provide an improved method and apparatus for selectively varying the magnitude of a bias applied to predetermined electrodes of an electrostatic recorder and thereby selectively vary the electric field in the printing gap.

In achieving the above-listed objects, applicant has devised a method for controlling the printing contrast of an electrostatic printer having at least one initiating, one printing, and one backing electrode comprising the steps of varying the magnitude of the bias applied across the printing gap between the printing electrodes and the backing electrode while maintaining constant the magnitude of the printing pulse potentials applied across the respective initiating and printing electrodes and while holding the backing electrode at a reference potential.

Other objects and aspects of the present invention may best be understood by referring to the following detailed description in conjunction with the accompanying figures wherein:

FIG. 1 is a perspective view of a portion of a print station with which is included a block diagram of circuitry for controlling, in accordance with applicants invention, the density of charge deposited on a record medium during electrostatic recording;

FIG. 2 is a diagrammatic view of the printing system of FIG. 1 illustrating the circuitry for providing the electrode bias and pulse control therefor;

FIG. 3 is a circuit diagram illustrating a preferred embodiment of applicants contrast control circuit;

FIG. 4 is a schematic diagram of a portion of an electrostatic recording station including the incorporation of the embodiment of applicants contrast control circuit shown in FIG. 3; and

FIG. 5 is a schematic diagram illustrating another embodiment of applicants contrast control circuit.

Referring to FIG. 1, there is shown an enlarged perspective view of an electrostatic recording station similar to that illustrated in the aforesaid patent which incorporates applicants contrast control circuitry. The recording or printing station comprises a transducer 11 including a plurality of initiating electrodes 13a, 13b, 13c, and 13d, which may be pin-shape in formation and arranged in a row and an extended printing electrode 15 cooperating therewith and assuming a bar-shape configuration. For purposes of illustration, the material which would normally form a supporting body for the transfer head is not shown. Spaced a substantially uniform distance from the initiating and printing electrodes is a backing or anvil electrode 17. A record medium 19 is supported on the backing electrode 17 and arranged, by means not shown, to be transported past the printing station. The record medium may comprise, for example, a charge-retentive or dielectric layer 21 and an electrically conductive backing layer 23.

Connected to the respective initiating electrodes 13a, 13b, 13c, and 13d are individual initiating pulse drivers 25 for selectively applying initiating voltage pulses to each initiating electrode. In like manner, a print pulse driver 27 is connected to printing electrode 15. The printing and initiating electrodes are spaced apart by suitable insulating material, such as described in the aforesaid patent. Contrast control means 29 is connected with the respective pulse control means and the backing electrode and is arranged, as hereinafter fully explained, to vary the bias applied across the respective printing electrodes and the backing electrode without affecting or altering the fixed bias applied in parallel across the respective initiating electrodes and their cooperating printing electrode and without affecting or altering the amplitude of the coincident voltage pulses applied to the print and print initiating electrodes.

In operation, an electrostatic latent image 31 is deposited on dielectric surface 21 of the record medium 19 when coincident initiating and printing pulses are applied to the respective initiating and printing electrodes. When such coincident pulses are sufiicient to cause ionization, the resulting charged particles create an avalanche of charged particles in the gap between the printing electrode 15 and the backing electrode 17 which is held at a reference potential, for example, ground.

A voltage ditference in the order of 1,600 volts is sufiicient to cause an ionizing spark between respective initiating and printing electrodes of electrostatic print heads known in the art. A single pulse applied to either printing electrode 15 or to one of the initiating electrodes 13a, 13b, 13c, and 13d is insufficient to cause a disruptive spark. However, coincidentally applied pulses from the respective printing pulse driver 27 and one or more of the initiating pulse drivers 25 are sufiicient in combination with the hereinabove mentioned bias to cause a disruptive spark, thereby initiating a printing operation. The strength of the electric field bias between the printing electrode 15 and the backing electrode 17, is insufiicent to introduce charged particles into the gap by field emission.

A discharge between a selected initiating electrode and the printing electrode introduces a large number of ions into the electric field in the space between the electrodes 13 and 15 and the backing electrode 17. The charged particles thus generated gravitate or are attracted toward the backing electrode under the effect of the electric field between the printing elecrode and the backing electrode, thus resulting in the disposition of electrically charged image areas 31 on the record medium.

The contrast control means 29 being coupled to the source of potential utilized to bias the respective initiating, printing, and backing electrodes, determines the potential difference which exists between the printing electrode and the backing electrode. If the potential of a pinbar discharge is made more positive with respect to the record medium, i.e. if the printing electrode is biased by the contrast control means 29 more positively with respect to the backing electrode, which is normally held at a reference potential, for example, ground, then the number of positive ions deposited on the paper, with the other parameters of the discharge remaining constant, will be increased. Conversely, if the potential pin-bar discharge is made less positive with respect to the record medium, then the quantity of positive charge deposited on the paper will decrease. As is known in the art, the amount of ink which will adhere to the record during the inking operation is proportional to the quantum of charge deposited on the record during the printing operation. Thus by controlling the potential of the ion source with respect to the record medium, the density of the deposited charge, and thereby the contrast of the developed latent image, may be selectively controlled.

Referring now to FIG. 2, there is shown a diagrammatic representation of a portion of an electrostatic printer and the electrode bias and drive control circuits which are commonly utilized to initiate a printing operation. As is known in the art, a potential difference sufficient to cause a spark between an initiating electrode 13 and its associated printing electrode 15 will serve to initiate non-contact electrostatic recording.

As shown in FIG. 2, separate pulse control means 35 and 37 may be utilized selectively to apply, for example, pulses to the control grids 39 and 41 of triodes 43 and '45, thereby inducing a printing pulse into the secondary windings 47 and 49 (of respective transformers 51 and 53. In FIG. 2, secondary windings 47 and 49 are connected respectively to a pin electrode 13 and a cooperating bar electrode 15. As also shown in FIG 2, a potential source is connected through the transformer secondaries to the cooperating pair of pin and bar electrodes 13 and 15 and functions to supply a continuous potential difference therebetween which is insufficient in and of itself to produce ionization of the atmosphere therebetween. However, upon the application of coincident pulses to the pin and bar electrodes from the trans-formers 51 and 53 a suflicient potential will be developed which in combination with the fixed bias applied by the potential source 55 will cause ionization between the print and bar electrodes. As zturther shown in FIG. 2, there is provided a second source of potential 56 which is designed to apply a variable potential difference between each bar electrode 15 and pin electrodes 13 considered as a heterogeneous electrostatic pole with respect to the backing or anvil electrode 17 considered as the other electrostatic pole. It is designed so as to provide a convenient, low cost method of and apparatus for altering the electric field in the printing gap for modifying the intensity of the electrostatic charges impressed on the record medium without adversely laifecting the operation of pulsing of the pin and bar electrodes for producing these impressed charges.

Referring now to FIG. 3, there is shown a schematic circuit diagram of a preferred embodiment of the invention. As hereinabove stated, the initiating and printing electrodes are connected to a common source of bias potential through the secondaries of their respective pulse transformers. For purposes of illustration, FIG. 3 omits the primary windings and pulse sources but does show the secondary windings 47 and 49 of the respective pulse transformers associated with a plurality of initiating and printing electrodes. It is to be understood that, as shown in FIG. 1, a plural number of pin electrodes are cooperatively associated with an individual one of the elongated printing electrodes. Connected across the source of bias potential 55 which, for example, may comprise a battery in the order of 300 to 600 volts, is a second variable source of potential equivalent to the source 56 of FIG. 2 which is illustrated as a potentiometer 57 having its movable contact arm 59 connected to a source OEf reference potential, for example, ground. The potentiometer 57 provides the variable contrast control. A by-pass capacitor 61 may be connected between one terminal of the potentiometer and the ground reference potential in order to eliminate any voltage transients arising from a capacitive imbalance between the respective pin and bar electrodes.

FIG. 4 is a schematic diagram of an electrostatic recording station which incorporates the contrast control circuitry of FIG. 3 as a part thereof. The printing station is illustrated as being composed of an array of initiating electrodes 13 arranged in row groupings with each row cooperating electrically with a printing electrode 15, and further including a backing electrode means 17 which is spaced apart from the electrode matrix array to define the printing gap. For purposes of illustration, the electrostatic recorder of FIG. 4 is shown as having two printing electrodes and two rows of initiating electrodes. However, as those skilled in the art are aware, the array may comprise a large number of groups of initiating and printing electrodes arranged in an elongated printing head, for example, such as that shown in FIGS. 5 and 6 of the aforesaid patent to Benn et al. In the gap between the electrode matrix array and the base electrode 17, the record medium 19 is disposed. The medium may be elongated in one dimension as shown and advanced in that dimension by means of rollers 70.

Each initiating electrode 13 of FIG. 4 is connected to one terminal of the constant bias potential source 55 through its secondary winding 47 of associated pulse transformer 51. The primary winding 63 of each pulse transformer 51 is connected toan appropriate pulse generating means 67. For purpose of clarity, certain of the primary windings and pulse generators are omitted from FIG. 4. Similarly, each printing electrode 15 is connected to the other terminal of the source of fixed bias potential 55 through its secondary winding 49 of associated pulse transformer 53. The primary winding 65 of each pulse transformer 53 is connected to a suitable pulse generator 69. The backing electrode 17 is connected to a source of reference potential, such as ground. Fhe variable contrast control means in the form of the potentiometer 57 is connected across the terminals of the constant bias potential source 55. The potentiometer is operable to vary the bias applied across the printing electrode 15 and backing electrode 17 in accordance with the positioning of movable contact arm 59, while the magnitude of the voltages applied to the initiating electrodes 13 and cooperating printing electrodes 15 is maintained constant, i.e., is unaffected by the positioning of arm 59.

In the operation of the system in FIG. 4, a disruptive discliarge, or a non-disruptive discharge of sufficient intensity, is s't-ruck between one or more initiating electrodes 13 and their associated printing electrode 15 when pulses are simultaneously applied to their respective transformers 51 and 53. By selectively varying the position of movable contact arm 59 of potentiometer 57, the magnitude of the bias applied to the printing electrodes 15 with respect to the source of reference potential may be varied whereby the intensity of the electric field in the gap may be altered with respect to the source of reference potential. As would be evident to those skilled in the art, the movable arm 59 may be selectively positioned either manually or automatically as a function of a predetermined system parameter.

Another form of contact control circuitry which may be employed to selectively vary the potential or a pinbar discharge with respect to the potential applied to the backing electrode is shown in FIG. 5. This figure illustrates an embodiment of the invention comprising first and second sources of constant bias potential 71 and 73, each being of approximately equal magnitude and having unlike terminals directly connected to a source of reference potential, for example, ground. The other unlike terminals of the respective sources of bias potential are connected through potentiometers 75 and 77 to the source of reference potential. The output of the contrast control circuit is taken across the respective movable cont-act arms 79 and 81 of the potentiometers 75 and 77 and, for example, may be connected into the electrostatic printer system illustrated in FIG. 4 in place of the bias and contrast control circuit there shown. For example, the terminals 97 and 99 of the circuit in FIG. 5 could be connected to the correspondingly numbered terminals of the system disclosed in FIG. 4. The movable contact arms 79 and 81 of potentiometers 75 and 77 are coupled by suitable mechanical linkage, represented by dotted line 95, such that the bias applied by the potential sources 71 and 73 through the respective movable contact arms to the pin and bar electrodes 13 and 15 respectively will remain constant while the magnitude of the potential for contrast control applied to the respective output terminals 97 and 99 may be varied with respect to the reference potential, in this instance, ground.

Another feature of the invention is the employment in the contrast circuitry of means for altering the polarity of the electric field established between the heterogeneous assembly of initiating and printing electrodes and the backing electrode. Such provision aids in sharply extinguishing the deposition of electrostatic charges on the dielectric surface and serves to sharply contrast the developed visible image from the background color of the record medium. As shown in FIG. 4, the contrast control circuitry is provided with means, generally identified at 101, for changing or switching the electric field in the gap from a polarity aiding in the deposition of electrostatic charges to the opposite polarity which has the effect of immediately inhibiting such action. The establishment of a field of opposite polarity in the gap will instantly cut off or extinguish the avalanche of charged particles propagating across the gap and will nullify any continued ionizla'tlion activity by .the initiating and printing electrodes.

The field polarity switching means 101 may take several forms. It is herein schematically represented by a reversing switch 103 having a movable member 105 which in one position applies the source of potential 55 across the potentiometer 57 in the direction to establish the biasing electric field in the gap aiding the deposition of charges and in the other position reversing the terminals of the potential source 55 with respect to the potentiome'ter and thereby reversing the polarity of the electric field in the gap. The result is that regardless of the operation of the initiating and printing electrodes 13 and 15 respectively as ion sources, the reversal of the polarity of the electric field in the gap will instantly cut oil? or extinguish the avalanche of charged particles propagated across the gap for deposition on the dielectric surface. Although the extinguishing polarity is shown in FIG. 4 to be equivalent to the normal biasing potential, the control device 101 may be designed, if desired, to supply a less avalanche extinguishing potential when the switching member 105 is moved to that position. It is thus apparent that by the provision of means 101 for reversing the polarity of the electric field in the gap, it is possible to control the deposition of electrostatic charges on the dielectric medium and to so deposit these charges in discrete areas on the medium as to produce a graphic display which will become visible when the electrostatic images are later developed by an inking agent.

Thus, by employing the invention in an electrostatic printer, it is possible to selectively vary the potential of a pin bar discharge with respect to the potential of the backing electrode Without altering the magnitude of the fixe'd or constant bias or the amplitude of the printing and initiating pulses. It is thus possible to selectively control the density of charge deposited on a record medium during electrostatic printing without adversely affecting the initiation of the printing operation.

It has been found in representative embodiments of the contrast control circuitry that for printing frequencies in the order of 1,000 characters per second, the current observed in return to the ground system is in the order of 100 to 300 m icroamp s. Because of the low order of the current involved, a fairly high resistance potentiometer may be employed in applicants contrast control circuitry, except in the circuit configuration shown in FIG. 5.

It should be understood that in lieu of a battery for the fixed bias 55 and a potentiometer for the variable potential 56 such sources of potential may be embodied by voltage regulated power supplies. Alternatively, a fixed preset power supply may be used for the bias source 55 and a programmed source of potential, or one varying proportionately to some input parameter, may be used in place of the potentiometer 57.

In describing the manner of establishing the electrically charged areas on the record medium 19, the backing or anvil electrode 17 has been indicated as a stationary electrically conductive supporting member and being at a reference potential. In the use of this invention it is not essential that the backing electrode be an electrical conductor. This is true when the backing layer 23 of the recording medium is itself a sufiicient electrical conductor for this purpose. When this is the case, the function of the anvil 17 is to support the recording medium at a substantially fixed distance from the electrodes 13 and 15. In order to maintain the potential of the conductive layer of the recording medium substantially at reference potential, an electrical contact may be made to the conductive layer at some location relatively remote to the electrostatic recording station.

As would be evident to those skilled in the art, minor modifications may be incorporated to adapt applicants invention to various electrostatic recording techniques and therefore it is applicants intention to be limited only as indicated by the scope of the following claims.

What is claimed is:

1. In apparatus for electrostatically recording information on a printing medium, the combination comprising:

a plurality of extended printing electrodes,

a plurality of groups of initiating electrodes, each group of initiating electrodes being cooperable with a predetermined printing electrode and spaced apart therealong,

cooperating anvil electrode means common to said printing electrodes and spaced therefrom to define means for translatably supporting a printing medium in said gap,

a plurality of pulse generator means for producing electrode voltage pulses,

first circuit means including a plurality of pulse transformers for individually connecting said plurality of pulse generator means to predetermined ones of said initiating and printing eelctrodes,

second circuit means for connecting a source of reference potential to said anvil electrode means,

third circuit means for applying a fixed bias from a potential source across each initiating electrode and its associated printing electrode, the magnitude of said bias being insufficient for electrostatically recording on said medium in the absence of the application of time-coincident electrode voltage pulses to selected initiating and printing electrodes,

means for causing selected ones of said plurality of pulse generator means to produce electrode voltage pulses, the amplitude and polarity of the electrode voltage pulses applied to selected initiating and print- 8 ing electrodes being sufiicient in combination with said bias to electrostatically record on a printing medium in the gap, and

contrast control means comprising potentiometer means connected across said fixed bias source of potential, said potentiometer having mova ble contact means connected to said reference potential and operable upon movement to vary selectively the magnitude of the bias coupled by said third circuit means to said printing electrodes with respect to the reference potential on said anvil electrode means while simultaneously maintaining the bias applied across said initiating and printing electrodes constant, said potentiometer means including a bypass capacitor means coupled between said fixed bias source and said reference potential.

2. In apparatus for electrostatically recording information on a printing medium, the combination comprising:

a plurality of extended printing electrodes,

a plurality of groups of initiating electrodes, each group of initiating electrodes being cooperable with a predetermined printing electrode and spaced apart therealong,

cooperating anvil electrode means common to said printing electrodes and spaced therefrom to define a p means for translatably supporting a printing medium in said gap,

a plurality of pulse generator means for producing electrode voltage pulses,

first circuit means including a plurality of pulse transformers for individually connecting said plurality of pulse generator means to predetermined ones of said initiating and printing electrodes,

second circuit means for connecting a source of reference potential to said anvil electrode means,

third circuit means for applying a fixed bias from a potential source across each initiating electrode and its associated printing electrode, the magnitude of said bias being insufiicient for electrostatically recording on said medium in the absence of the application of time-coincident electrode voltage pulses to selected initiating and printing electrodes;

means for causing selected ones of said plurality of pulse generator means to produce electrode voltage pulses, the amplitude and polarity of the electrode voltage pulses applied to selected initiating and printing electrodes being sufiicient in combination with said bias to electrostatically record on a printing medium in the gap, and

contrast control means comprising a plurality of potentiometers disposed in parallel across said fixed bias source of potential, each potentiometer having at least one movable contact means connected to said reference potential and operable upon movement to vary selectively the magnitude of the bias coupled by said third circuit means to said printing electrodes with respect to the reference potential on said anvil electrode means while simultaneously maintaining the bias applied across said initiating and printing electrodes constant and mechanical linkage means for maintaining a predetermined positional relationship between said several movable contact means.

3. In apparatus for electrostatically recording information on a-printing medium, the combination comprising:

a plurality of extended printing electrodes,

a plurality of groups of initiating electrodes, each group of initiating electrodes being cooperable with an individual one of said printing electrodes and spaced apart therealong,

cooperating backing electrode means common to said printing electrodes and spaced therefrom to define means for translatably supporting a printing medium in said gap,

a plurality of pulse generator means fior producing electrode voltage pulses, first circuit means including a plurality of pulse transformers for individually connecting said plurality of pulse genenator means to individual ones of said initiating and printing electrodes, second circuit means for connecting said backing electrode means to a source of reference potential, combination biasing and contrast control means including first means for applying a fixed magnitude bias potential across said initiating and printing electrodes, second means for applying a bias potential across the printing electrodes and the backing electrode means of a magnitude insuflicient to electrostatically charge a printing medium in the gap, and means for adjusting said last-mentioned bias potential to vary the strength of the electrostatic field in said gap, including first and second sources of bias potential, first and second potentiometer means connected across the respective terminals of said first and second bias potential applying means, each potentiometer means having a movable contact means, mechanical linkage means connecting the respective movable contact means of said first and second potentiometer means and maintaining a predetermined positional relationship therebetween, means for coupling unlike terminals of said first and second bias applying means to said reference potential, and means for coupling said respective contact means adjusting said bias potential between said printing electrodes and said backing electrode means includes electrostatic field polarity switching means for instanly exting uishing electrostatic recording on said printing medium and thereby increasing the charge difierential between charged and uncharged areas on said printing medium.

References Cited UNITED STATES PATENTS 2,777,957 1/1957 Walkup 25049.5 2,817,765 12/1957 Hayfond 34674 2,879,395 3/1959 Walkup 34674 3,004,819 10/1961 Anderson 34674 3,068,479 12/1962 Benn 34674 3,307,198 2/1967 Morgan 34674 3,358,289 12/1967 Polee 34674 3,372,400 3/1968 Epstein 34674 BERNARD KONICK, Primary Examiner. LEE J. SCHROEDER, Assistant Examiner.

US. Cl. X. R.

PO-IOSO (5/69) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. Dated June 10, 1 969 Inventor(s) David Starr, Jr.

It is certified that error appears in the aboveidentified patent and that said Letters Patent are hereby corrected as shown below:

[- Column 1, line 62, 'eelctrode should read ---olectrode---. j

Column 2,. line 34, after "ionization" add a period; Column 5, line 73, Contact" should read ----contrast---. Column 6,

line 28, after contrast" insert --control----. Column 7, line 62, "eelctrodes" should read -electrodes- Column 10, line 14, instenly' should read ----J'.nstantly---.

SIGNED AN'D SEALED MAR 2 -1970 (SEAL) Attest:

Edward M" Fletcher, Jr.

m WILLIAM E. SQHUYLER, JR- Amslmg Officer Commissioner of Patents 

